Previously, the only means of obtaining a continuously variable RF time delay was by mechanical methods such as line stretchers. Those methods to steer phased array antennas used electronic phase shifters. They provide RF phase shifts which are independent of the RF frequency. This unwanted behavior results in a beampointing error known as squint, where the different frequency components of a modulated RF carrier will point in different directions. Elimination of this error requires true time delay, where the RF phase shift is linearly proportional to RF frequency. Discrete delay using optical switching circuits has been thoroughly investigated by many researchers but the result has always been the problem of how to deal with the robust power requirements and significant loss of the many optical switches which are required for high resolution delay capability. By using the spatial signal processing technique of the design presented here, continuously variable RF delay is now possible for the first time in an efficient optical architecture.
The phased array application discussed here is a specific example of the general concept of transversal filtering. Thus the integrated transversal filter architecture presented enjoys a wealth of other applications in the field of microwave signal processing.
Conventional electronic methods for generating the necessary phase information for dynamically steered antennas tend to be lossy and inefficient. Also the group delay is not adjustable utilizing these electronic phase shifters. Other optical methods for realizing variable group delay include switched fiber schemes. This technique requires many optical switches to achieve high resolution RF radiation patterns. These optical switches also require enormous power, and tend to be very lossy, hence the lack of a practical implementation to date.
Currently, microwave transversal filtering is achieved using tapped transmission line architectures. These suffer from the same deficiencies as with the phased array applications. Also, the delays are fixed, not variable. The variable delay line presented here now allows for an efficient and reconfigurable microwave adaptive filter.